1. Technical Field
The invention relates to a method and a device for the production of castings from aluminum hand magnesium alloys and to the use of an induction crucible furnace with sufficient melting and heat retention capacity for the production of castings, in particular from particle-reinforced aluminum and magnesium alloys.
2. Relevant Art
In the casting of particle-reinforced aluminum and magnesium alloys, there is the problem of keeping the particles in suspension in the melt, in order to obtain a homogeneous distribution of the particles in the casting. For this purpose, a continuous movement of the melt is necessary, which prevents the particles from settling.
This is counteracted by the attempt to remove gas and oxide inclusions from the melt or to avoid such inclusions, this being capable of being achieved essentially by keeping the melt as steady as possible and by avoiding turbulences during the casting operation. The low-pressure chill-casting method with fossil or resistance heating or an induction channel is particularly suitable in this respect, since the melt can be steadied in a corresponding furnace and be pressed at low velocity by means of gas pressure through a riser penetrating deep into the melt, into a chill or mold arranged above the melt.
From the paper: xe2x80x9cErfahrungeneiner bsterreichischen Leichtmetallgiesserei mit dem Niederdruck-Kokillengussxe2x80x9d [xe2x80x9cExperiences of an Austrian light-metal foundry with low-pressure chill castingxe2x80x9d] by Johann Berger, appearing in xe2x80x9cGiessereixe2x80x9d [xe2x80x9cCastingxe2x80x9d], Sep. 21, 1961, and from the paper: xe2x80x9cDas Niederdruck-Kokillengiessverfahren in heutiger Sichtxe2x80x9d [xe2x80x9cThe low-pressure chill-casting method at the present timexe2x80x9d] by Johann Berger, appearing in xe2x80x9cGiessereixe2x80x9d [xe2x80x9cCastingxe2x80x9d], Feb. 13, 1969, it is clear that the principle of the inductive melting can be adopted in the low-pressure casting of aluminum alloys. The smelting plants described are in each case operated on the double-crucible principle. The electromagnetic field of the (supply-frequency) inductor leads mainly to the heating of only the outer cast-iron crucible. As a result of heat conduction, the graphite crucible located in the cast-iron crucible is likewise heated, with the result that the batch located in this crucible is melted down, in that a transport of energy from the graphite crucible into the metal takes place.
In this application, the cast-iron crucible has the function of being heated as a result of the electromagnetic field of the coil, for which purpose it is intentionally designed to be thick-walled, in order to conduct this induced heat further on to the graphite crucible. The graphite crucible has the function of protecting the aluminum melt from undesirable reactions with the cast-iron crucible. On account of the high outlay, this concept does not constitute the ideal solution and has not found widespread use. The thick-walled cast-iron crucible has the effect that the batch is shielded against the induced currents, so that the bath movement is not sufficient to make it possible to keep the particles in suspension when particle-reinforced light-metal alloys are being melted.
Developments throughout the world are seeking suitable methods and processes for melting and casting particle-reinforced aluminum alloys and are aimed at also using inductive melting in conjunction with suitable crucible materials in order to generate in the melt bath a flow pattern which may be sufficient to keep particles of specific characteristics in suspension.
European Patent Application 0 662 361 and German Laid-Open Publication DE 196 26 175 A1 describe methods and devices for the casting of particle-reinforced aluminum alloys, in which a filling tube with a filling piston is arranged below a mold. A preportioned quantity of casting material is introduced into the filling tube and is subsequently melted down or kept hot by induction; swirling of the melt together with the particle reinforcing material being achieved by virtue of the induction. These are in each case individual melting methods having the disadvantage that the metal to be melted has to be portioned for the respective casting and the method is restricted in terms of its melting capacity, so that material-intensive castings therefore cannot be manufactured. Nor is the productivity of these methods mentioned very high, since the cycle time is composed of the melting time and the solidifying time and is consequently low. In contrast to low-pressure casting, the actual casting is carried out by means of a mechanical piston which presses the melt or the pasty material into the mold from below.
In the lost crucible process (LOC), a fiber crucible is employed, which is destroyed during casting as a result of the mold being filled from below and can consequently be used only for second casting, the efficiency of the method being impaired as a result.
The problem on which the invention is based is to achieve as high process reliability as possible, a rapid cycle time and a high output in the production of castings from particle-reinforced aluminum and magnesium alloys, while at the same time ensuring that the material of the castings are as homogeneous as possible. This aim is achieved by the use of an induction crucible furnace for receiving a batch stock which is sufficient for a relatively large number of casting operations, with a lining or a crucible consisting of electrical nonconductive refractory material, with direct However, other casting methods may also be employed, such as, for example, diecasting, squeeze casting, rheo casting and the like.
The invention affords the following advantages, as compared with the prior art:
1. Complicated treatment of the melt by means of mechanical agitation can be dispensed with, as compared with the conventional melting technique for particle-reinforced material.
2. Process management is such that gas and oxide inclusions are reduced, as compared with the previous melting and casting technique for particle-reinforced alloys.
3. As compared with casting with a lost crucible, costs for the crucibles are saved, and the method is more flexible, since the preliminary material used does not have to be only continuous-casting billets.
On the basis of this set problem, it is proposed, furthermore, according to the invention, to use an induction crucible furnace for the manufacture of castings from particle-reinforced aluminum and magnesium alloys, with direct inductive heating of the batch and with an electrically nonconductive refractory lining or crucible.
Induction crucible furnaces with direct inductive heating of the batch contain a batch stock which is sufficient for a relatively large number of casting operations. It has been shown, surprisingly, that, although the movement of the melt brought about by inductive heating is sufficient to keep the particles in suspension in the melt, this movement of the melt is not so intensive that gas or oxide inclusions occur to an increased extent.
The induction crucible furnace may be designed in such a way that the batch in the form of slabs, billets or preliminary materials is introduced and is melted down, so that the induction crucible furnace is used both for melting and for heat retention, or in such a way that preliminary material which is already liquid is introduced and is kept hot and agitated in the induction crucible furnace.
The induction crucible furnace may be designed in such a way that it receives the batch directly, but it is also possible to insert a crucible for the batch into the induction crucible furnace. It is essential, in both cases, that the lining of the induction crucible furnace or the inserted crucible consist of a nonmagnetic, electrically nonconductive, refractive material, so that the generation of heat takes place directly in the batch.
So that the gas pressure necessary for the low-pressure casting method can be applied to the melt surface, the furnace space is closed by means of a closing plate which is sealed off either relative to the induction crucible furnace or relative to the crucible inserted into the furnace space.
The riser fastened to the closing plate consists of nonmagnetic material and may be arranged centrally or preferably in the vicinity of the wall in the region of high induction, so that the field forces of the induction coil ensure an intermixing of the melt even in the riser and the particles remain in suspension even in the riser.
The induction coil may be designed as a full or short coil or so as to consist of a plurality of part coils and may be arranged and connected in such a way that it can be switched on for melting down, for example over the entire height of the furnace space, and for heat retention and agitation only in the lower region of the furnace space.
Furthermore, it is possible to subject the induction coil to a high frequency for melting down, but to a low frequency for heat retention and agitation. Preferably, in this case, the melting-down frequency may be above 500 Hz and the heat retention and agitation frequency may be equal to or lower than 50 Hz.
The casting operation, which is as free of turbulence as possible, can be assisted by subjecting the casting mold to a vacuum, so that only a relatively low excess pressure needs to act on the melt surface.